fix: clear leaked running loop in MCP client background thread (#2111)

Author: mkmeral

src/strands/tools/mcp/mcp_client.py

@@ -835,6 +835,9 @@ def _background_task(self) -> None:
         This allows for a long-running event loop.
         """
         self._log_debug_with_thread("setting up background task event loop")
+        # Clear any running-loop state leaked by OpenTelemetry's ThreadingInstrumentor, which wraps Thread.run()
+        # and can propagate the parent thread's event loop reference, causing run_until_complete() to fail.
+        asyncio._set_running_loop(None)
         self._background_thread_event_loop = asyncio.new_event_loop()
         asyncio.set_event_loop(self._background_thread_event_loop)
         self._background_thread_event_loop.run_until_complete(self._async_background_thread())

tests/strands/tools/mcp/test_mcp_client_contextvar.py

@@ -88,3 +88,39 @@ def capturing_background_task(self):
     )
     # Verify it was indeed a different thread
     assert background_thread_value["thread_id"] != main_thread_id, "Background task should run in a different thread"
+
+
+def test_mcp_client_clears_running_loop_in_background_thread(mock_transport, mock_session):
+    """Test that _background_task clears any leaked running event loop state.
+
+    When OpenTelemetry's ThreadingInstrumentor is active, Thread.run() is wrapped to propagate
+    trace context, which can leak the parent thread's running event loop reference into child
+    threads. This causes "RuntimeError: Cannot run the event loop while another loop is running"
+    when the background thread calls run_until_complete().
+
+    This test simulates that scenario by setting a running loop before _background_task runs
+    and verifying it gets cleared.
+    """
+    import asyncio
+
+    cleared_running_loop = {}
+
+    original_background_task = MCPClient._background_task
+
+    def simulating_otel_leak_background_task(self):
+        # Simulate OTEL ThreadingInstrumentor leaking the parent's running loop
+        fake_loop = asyncio.new_event_loop()
+        asyncio._set_running_loop(fake_loop)  # type: ignore[attr-defined]
+
+        # Call the real _background_task — it should clear the leaked loop and succeed
+        try:
+            return original_background_task(self)
+        finally:
+            cleared_running_loop["success"] = True
+            fake_loop.close()
+
+    with patch.object(MCPClient, "_background_task", simulating_otel_leak_background_task):
+        with MCPClient(mock_transport["transport_callable"]) as client:
+            assert client._background_thread is not None
+
+    assert cleared_running_loop.get("success"), "_background_task should have run successfully despite leaked loop"